1. Field of the Invention
The present invention relates to a metal pattern forming method, and in particular, a method for forming a metal pattern by depositing an acetylene compound and an ink liquid containing a metal salt onto a substrate, and synthesizing a metal acetylide on the substrate.
2. Description of the Related Art
In general, a photolithography method (also referred to simply as “photolitho”) has been used as a method of forming a metal pattern. In the photolithography method, a resist layer is formed of a dry film resist or liquid resist on a copper clad laminated plate constituted of an insulating layer and copper foil. This resist layer is then irradiated with ultraviolet light through a photomask, and then developed, thereby patterning the resist layer. Thereupon, a metal pattern is obtained by removing the copper foil which is not covered with the patterned resist layer, by means of etching with phosphoric acid, or the like. This method is referred to as a subtractive method. Apart from this method, it is also possible to form a metal pattern by means of a semi-additive method or an additive method. In the semi-additive method or the additive method, a photoresist is patterned in a similar way to the subtractive method, and the patterned photoresist is used as a plating mask.
However, photolithography involves various issues, such as the fact that costs are generally high because it takes a long time of one month or so to make a photo mask, the fact that a large volume of chemicals which are considered to present a considerable environmental burden are used, such as etching liquid, developer liquid, resist stripper, and the like, and the fact that all of the processes are redundant, and so on.
In view of this, Japanese Patent Application Publication No. 2002-134878 discloses a method of forming a metal pattern by patterning an ink of metal particles on a substrate by means of an inkjet or other printing method and then carrying out electroplating. However, the method described in Japanese Patent Application Publication No. 2002-134878 uses a silver colloid dispersion and hence there are concerns with regard to the adhesiveness to the substrate. Furthermore, in order to generate a catalyst, the dispersant on the surface of the silver micro-particles is defatted, but a long heating step of 15 minutes at 150° C. to 300° C. is required for this purpose. Hence, there is a problem in that this method is difficult to apply to a substrate which has poor resistance to heat.
Furthermore, Japanese Patent Application Publication No. 1-131250 discloses a method of polymerizing a metallic compound by heating a metal acetylide compound or by heating an acetylene compound in the presence of a metal element. This method is extremely stable, and has excellent solubility and controllability of the film thickness. This method can be used to form a catalyst film for electroless plating or a conductive film. Moreover, catalytic activity is obtained in a relatively low temperature process without using palladium which is highly expensive, and the adhesive force with respect to the substrate is strong since a chemical change occurs in the heating step and the material reacts with the substrate. Furthermore, a processing step of several seconds to approximately one minute at 100° C. to 180° C. is sufficient, and hence this method can be applied to a substrate having poor resistance to heat.
Moreover, Japanese Patent Application Publication No. 2001-154215 discloses a method of manufacturing a conductive film by applying an application liquid containing a metal acetylide to a supporting body, forming a conductive layer by means of heat treatment, and forming a plating layer on this conductive layer.
In Japanese Patent Application Publication No. 2001-154215, a metal acetylide solution (for example, a silver acetylide solution) is applied by a bar coater to form a conductive layer. However, if the metal acetylide solution could be deposited onto the substrate in the form of droplets by means of an inkjet apparatus, then a great merit would be obtained in that a metal pattern could be printed directly onto the substrate.
There is no problem if using an application apparatus, such as a bar coater, as an apparatus for applying a metal acetylide solution onto a substrate, as in Japanese Patent Application Publication No. 2001-154215. However, if it is sought to print a metal pattern, such as a pattern for a plating catalyst or a conductive pattern for an electrical circuit, or the like, directly onto a substrate by depositing metal acetylide solution from the nozzle section of an inkjet apparatus as the depositing apparatus, then the following problems occur.
(1) If a σ complex is used as a silver salt monomer, then the monomer is precipitated by liquid evaporation and condensation in the nozzle section, and therefore nozzle blockages and ejection failures are liable to occur.
(2) If a π complex is used as a silver salt monomer, then since the π complex is thermally instable, it suffers severe decomposition upon assuming a solvent-free state, and therefore has not allowed stable use. Moreover, during inlet ejection, with the passage of time, the silver mirror is produced inside the head and the flow channel becomes sealed.
Furthermore, it has not been possible to eliminate general problems which are intrinsic to inkjet systems, namely, the problem of wetting and spreading of the deposited ink which prevents the formation of fine patterns, the mutual interference between deposited droplets and the consequent decline in patterning accuracy.